idp groupnames cannot contain `:`

given an idp-group with the literal name `GRP:foo` as used in the below config, the server will crash on startup:

```yaml
[/]
srv
accs:
A: @GRP:foo
```

**workaround:** until this is fixed, remap `:` to `/` or something else;

```yaml
[global]
idp-chsub: |:/|

[/]
srv
accs:
A: @GRP/foo
```

the crash is an errormessage complaining that the config syntax is invalid due to an exception from the following sanchk: https://github.com/9001/copyparty/blob/da6e2ddca96dffc4dbe53bda25d2034428fad3d0/copyparty/authsrv.py#L1521-L1522